III-V nanowires (NWs) are considered key elements for the fabrication of future nano-photonic devices. GaAs is a prime candidate for such applications. Recently, optically-pumped lasing has been reported for single GaAs-AlGaAs core-shell NWs. Indeed, the overgrowth of a wider bandgap AlGaAs shell around GaAs NWs leads to effective passivation of GaAs surface states and to improved minority carrier diffusion lenghts, and recombination lifetimes with respect to bare GaAs NWs. However, growth conditions (temperature, V:III precursor ratios in the vapor, etc.) and relevant geometrical parameters (namely, the shell-thickness to core-radius ratio) of GaAs-AlGaAs core-shell NWs affects the nanostructure carrier lifetime, as well as built-in lattice strain and core radiative emission. GaAs-AlGaAs core-shell NWs were grown by low pressure MOVPE in an Aixtron reactor, using trimethylgallium, trimethylaluminum and tertiarybuthylarsine as Ga, Al and As precursors. GaAs core NWs were grown at 400°C on either semi-insulating (111)B-GaAs or (111)Si substrates by the Au-catalyzed method, after which an Al0.33Ga0.67As shell was overgrown at 650°C by conventional MOVPE. In some cases a few-nm thin GaAs cap layer was deposited around the shell to protect it from oxidation. During NW core and shell+cap growth the V:III precursors ratio in the vapor was changed between 5:1 and 30:1. We demonstrate that, under conventional MOVPE growth conditions, the initial diameter, height and density of core NWs strongly impact on the actual shell growth rate. We further explain such findings by validating a detailed MOVPE growth model of AlGaAs shell based on the mass-transport of III-group species. Besides ensuring strict control over shell thickness in core-shell and core-multishell NW structures, the model allows to calculate effective (around the NWs) vapor stoichiometry during shell growth. The 7K photoluminescence of GaAs-AlGaAs core-shell NWs is thus reported as function of the NW relevant geometrical parameter, and values of V:III effective ratios during AlGaAs shell growth obtained from the model. After deconvoluting the strain-dependent red-shift of GaAs excitonic emission from the shell-dependent localization effect in present core-shell NWs, the latter is reported as function of as-calculated V:III effective ratios and discussed.
Advances in the growth and characterization of III-V core-shell nanowire arrays for photovoltaics and photonics
P Prete;
2014
Abstract
III-V nanowires (NWs) are considered key elements for the fabrication of future nano-photonic devices. GaAs is a prime candidate for such applications. Recently, optically-pumped lasing has been reported for single GaAs-AlGaAs core-shell NWs. Indeed, the overgrowth of a wider bandgap AlGaAs shell around GaAs NWs leads to effective passivation of GaAs surface states and to improved minority carrier diffusion lenghts, and recombination lifetimes with respect to bare GaAs NWs. However, growth conditions (temperature, V:III precursor ratios in the vapor, etc.) and relevant geometrical parameters (namely, the shell-thickness to core-radius ratio) of GaAs-AlGaAs core-shell NWs affects the nanostructure carrier lifetime, as well as built-in lattice strain and core radiative emission. GaAs-AlGaAs core-shell NWs were grown by low pressure MOVPE in an Aixtron reactor, using trimethylgallium, trimethylaluminum and tertiarybuthylarsine as Ga, Al and As precursors. GaAs core NWs were grown at 400°C on either semi-insulating (111)B-GaAs or (111)Si substrates by the Au-catalyzed method, after which an Al0.33Ga0.67As shell was overgrown at 650°C by conventional MOVPE. In some cases a few-nm thin GaAs cap layer was deposited around the shell to protect it from oxidation. During NW core and shell+cap growth the V:III precursors ratio in the vapor was changed between 5:1 and 30:1. We demonstrate that, under conventional MOVPE growth conditions, the initial diameter, height and density of core NWs strongly impact on the actual shell growth rate. We further explain such findings by validating a detailed MOVPE growth model of AlGaAs shell based on the mass-transport of III-group species. Besides ensuring strict control over shell thickness in core-shell and core-multishell NW structures, the model allows to calculate effective (around the NWs) vapor stoichiometry during shell growth. The 7K photoluminescence of GaAs-AlGaAs core-shell NWs is thus reported as function of the NW relevant geometrical parameter, and values of V:III effective ratios during AlGaAs shell growth obtained from the model. After deconvoluting the strain-dependent red-shift of GaAs excitonic emission from the shell-dependent localization effect in present core-shell NWs, the latter is reported as function of as-calculated V:III effective ratios and discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
